Wow. Where to start?

Pretty much anything that has gotten anywhere near Agama is bound to be excellent. It's hard to choose which one of Agama's tilesets is best, but I would likely have to go with Heaven. I also like Swamps of the Sleeping Jaguar, but Heaven is just outstanding. I also have a spot in my heart for Overlord's Top Secret ][. Also near the top of my list is MidN's School v2 and School v3 tilesets, CraccoBoy's Desert of the Lazy Donkies, etc.

There is no "best" tileset but I'll have to say that Agama is easily the best tileset creator of all time.